Liquefied gas such as liquefied natural gas (LNG) is obtained by liquefying a gas that is in a vapor state at a room temperature at an extremely low temperature that is lower than a saturation temperature, and is carried by a conveying unit such as ship.
In addition, a cargo tank for receiving the liquefied gas is provided in the ship.
The cargo tank has various types of thermal insulating structures in order to maintain the liquefied state of the liquefied gas at the extremely low temperature, for example, −163° C. or less, from departure place where the liquefied gas is injected into the cargo tank to destination where the liquefied gas is unloaded from the cargo tank. In addition, the cargo tank includes a liquefied gas leakage prevention structure for preventing the liquefied gas from leaking out of the cargo tank.
The cargo tank may be manufactured in various types, for example, a MOSS type of independence tank that is formed as a spherical metal structure or a membrane tank type formed to have a plurality of cell structures, according a shape and a structure of the cargo tank.
In particular, the cargo tank manufactured as the membrane tank type includes a main wall formed of stainless steel for surrounding a receiving space that is formed in the cargo tank to receive the liquefied gas, a thermal insulating panel assembly surrounding the main wall, and an outer wall surrounding the thermal insulating panel assembly.
In addition, the thermal insulating panel assembly is formed of a thermal insulating material such as polyurethane foams, and includes a plurality of first thermal insulating panels and a plurality of second thermal panels respectively disposed in two-layered structures, and auxiliary walls disposed between the first thermal insulating panels and the second thermal insulating panels and formed of a triplex material having a plurality of layers formed of, for example, aluminum and fiber glass.
Here, the plurality of first thermal insulating panels and the plurality of second thermal insulating panels are disposed alternately with each other. In addition, bridge pad is disposed between the plurality of second thermal insulating panels for filling separate spaces between the second thermal insulating panels.
In addition, the auxiliary walls disposed between the plurality of first thermal insulating panels, the plurality of second thermal insulating panels, and the plurality of bridge pad are fixed on the panels or the pads via an attachment method.
Here, the plurality of first thermal insulating panels disposed in a first layer of the thermal panel assembly and the plurality of second thermal insulating panels and the bridge pad disposed in a second layer of the thermal panel assembly overlap each other to certain regions.
On the other hand, the main wall and the thermal insulating panel assembly that are adjacent to the receiving space are exposed to the extremely low temperature, in a state where the liquefied gas is received in the receiving space of the cargo tank.
Therefore, the plurality of panels and the plurality of bridge pad forming the thermal insulating panel assembly are thermally contracted. Here, when the plurality of panels and the plurality of bridge pad are contracted in a state of overlapping each other to a predetermined degree, stress caused by the thermal contraction of the panels and the bridge pad is applied on the auxiliary walls fixed between the plurality of panels and the plurality of bridge pad.
In addition, since the stress is concentrated on boundaries of the panels and the bridge pad, the auxiliary walls may be broken by the concentrated stress, and thus, a sealing state of the liquefied gas may be damaged.